The surface finish on an ophthalmic lens is generally affected by two common types of surface defects inherent in a lens machining operation. The first type of defects is caused by the actual removal of material from the lens surface, and the second type is caused by the vibration of one or more of the machine's elements.
The major factors influencing surface defects of the first type are the outline of the cutting tool in contact with the lens, the microstructure and chemical composition of the material of the lens, and thermal effects occurring during the material-removal process. In this respect, improvement in surface finish may be obtained to various degrees by increasing the cutting speed, decreasing the feed of cut and changes in cutting fluid and tool geometry.
Surface defects of the first type generally have surface irregularities of a relatively small extent; usually less than 78 microinches (2 microns) peak-to-valley. This type of surface defects is usually entirely coverable by a clear resinous coating well known in the art of lens making.
Vibration of machine elements, however, is often the cause of deeper irregularities on the surface of generated lenses. Vibration of machine elements is very complex, and often has a combination of interrelated causes as explained below.
Rotating machines in general are known to vibrate according to three basic types of vibration. A first basic type of vibration is caused for example, by an unbalance of the tool, eccentricity in the tool chuck, and defects in a drive belt. This first type of vibration has an amplitude in a radial direction relative to the axis of rotation of the tool.
The second basic type of vibration in rotating machines has an amplitude in an axial direction along the axis of rotation of the tool spindle for example. This axial vibration is often caused by a vacillating plane of rotation of the tool's cutting face, or by intermittent cutting forces. Because the cutting force applied on a tool has a vectorial component in the axial direction, a vibration of the radial type also tends to cause a vibration in the axial direction.
The third basic type of vibration in rotating machines is a torsional vibration. This type of vibration is often associated with torque pulses inherent with some types of electric motors and motor controls. These pulses tend to alternatively increase and reduce the amount of material being removed from the surface of the lens during a full rotation of the tool. This torsional vibration, when cutting forces are present, is also known to cause vibration of the radial and axial types.
It is further known from the science of vibration analysis that any of the above three types of vibration can excite a resonant condition in the structure of the machine, with the consequence of increasing the amplitude of the vibrating element.
The problem of vibration in a lens generating apparatus has been partially addressed in the past. For example, a first apparatus for manufacturing ophthalmic lenses is described in U.S. Pat. No. 4,434,581 issued on Mar. 6, 1984 to Robert G. Spriggs. This apparatus has X-Y fluid-bearing tables mounted on a granite bed weighing about 4,000 pounds. This bed is undoubtedly particularly efficient for reducing the amplitude of a vibration in the radial direction, that is in a direction perpendicular to the table. However, it will be appreciated that a massive table is less appropriate for reducing vibrations of the axial and torsional types where the lens holder or the tool spindle is mounted at some distance for the surface of that table.
In another example, U.S. Pat. No. 4,760,672 issued on Aug. 2, 1988 to Darcangelo et al. describes a lens grinding and polishing apparatus wherein a drive motor is mounted remotely from the tool supporting table, and is driving a micrometer drive via a drive belt. This type of mounting isolates the tool and work spindles from the vibration of the drive motor.
Although the apparatus in both the above examples are most likely efficient in dampening the radial vibrations of the tool and work support spindles, these apparatus as well as other optical lens generating apparatus of the prior art lack vibration dampening properties in the axial and circumferential directions relative to the rotary tool and work holder.